1. Field of the Invention
This invention relates to a process for reducing the radioactive contamination in waste product phosphogypsum.
In the industrial production of phosphoric acid by wet methods, phosphate rock is reacted with concentrated sulfuric acid for simultaneously producing both phosphoric acid solutions and calcium sulfate products. The calcium sulfate, which may be recovered as dihydrate or phosphogypsum, hemihydrate, or anhydrite, is contaminated with most of the impurities originally present in the phosphate ore. In the past, these calcium sulfate products were generally discarded as unwanted by-products because of the impurities and large waste piles of these materials can be found at most phosphoric acid plants.
2. Description of the Prior Art
U.S. Pat. Nos. 3,949,047 and 4,282,192 disclose treating the monocalcium phosphate (MCP) solution during phosphoric acid manufacture prior to the precipitation of the calcium sulfate. In the former patent, the MCP solution is treated by adding barium compounds to reduce radium contamination, and in the latter patent, the MCP is treated with a sequestering agent and a first calcium sulfate precipitate high in radium is discarded. U.S. Pat. No. 4,146,568 discloses a process for reducing radioactive contamination in the phosphogypsum itself by slurrying it with a dilute sulfuric acid containing barium sulfate and separating the solids produced thereby into fine and coarse fractions. It discloses that the fine fraction predominates in the radioactive contamination. It does not disclose how much of the starting phosphogypsum is recoverable, but losses could be considerable. U.S. Pat. No. 3,984,525 discloses converting phosphogypsum by treating it with sulfuric acid at temperatures of about 75.degree. C. to obtain a total sulfate content in the slurry of about 7%. Calcium sulfate hemihydrate is recrystallized and separated from the phosphoric acid solution (about 33% P.sub.2 O.sub.5) whereby the concentration of the phosphoric acid is increased. This patent does not appear to disclose any reduction of radioactivity in the calcium sulfate products.
There is still a need in the art for an effective and economical means for removing a substantial portion of the radioactive material that is present in phosphate rock. There is a need for making calcium sulfate products that have acceptable levels of radioactivity such that they may be utilized in the manufacture of wallboard and other industrial and construction materials. There is also a need at the present time to provide improved processes for reducing the radioactive contaminants in waste product phosphogypsum, and particularly for such a process that may be performed at a location separate and away from the phosphate rock processing operations. The process of the present invention offers a solution to these needs.